The present invention generally relates to pressure control and, more particularly, to apparatus and methods for detecting excessive pressure leakage.
In flight, Cabin Pressure Control Systems (CPCSs) normally exhaust pressurized cabin air directly to atmosphere pressure.
On some aircraft, this pressurized cabin air is used to provide a very small amount of aft thrust by utilizing a thrust recovery type outflow valve that accelerates the cabin air exhaust to super-sonic pressure ratios and directs this exhaust as close to the aft direction as is geometrically possible—given the constraints of the valve installation. Thrust recovery valves are not aerodynamically efficient when full-open on the ground with a low pressure ratio, and thus are very large, heavy, and expensive. Further, thrust recovery valves operate with very high aerodynamic torques, causing large rotary actuator gear boxes and motors. This makes the rotary actuators heavy and expensive and requires higher-current motor controllers (which also dissipate more power and are large and expensive).
An alternative use of cabin air exhaust for its potential energy is considered where the cabin air exhaust is consumed directly by the APU core compressor. This pressurized air enables increased APU operating efficiency relative to the APU ingesting atmosphere air—especially when the airplane is in flight and the atmosphere air is at relatively (to the pressurized cabin) low pressure.
Use of cabin pressure air directly in the APU core compressor assumes that there is enough air conditioning (environmental control system, ECS) flow into the cabin to overcome the exhausts of cabin air through the APU, the outflow valves (OFV), and any natural leakage paths out of the pressurized fuselage:WECS≥WAPU+WOFV+WLEAKS
This relationship is required to pressurize the cabin safely while also providing adequate flow to the APU for its intended functions (output power).
Further, for a new or relatively new airplane, the fuselage leakage flow may be less than when the airplane is older and doors, window, and other seals age. For an older airplane when more cabin inflow is lost to fuselage leakage, flow to the APU may be reduced (by throttling the cabin exhaust flow to the APU) to ensure that the fuselage is adequately pressurized for occupant safety and comfort. Unfortunately, with a reduction in APU flow, the APU efficiency and ability to produce power can be adversely affected such that the overall system cannot fulfill its intended functionality.
Legacy systems (which do not feed the APU with cabin air) do not currently compute an absolute value for the fuselage leakage, although relative leakage can be considered too great if outflow valves are closed and the fuselage is unable to pressurize according to the expected schedule. However, this condition can also be caused by a reduction of ECS flow below the margin required to pressurize. Thus, without knowing the ECS flow, it is impossible to determine whether the loss of pressure is due to excessive leakage or low ECS flow.
A method to monitor the fuselage leakage on a legacy system (which does not feed the APU with cabin air) architecture is provided in McGill et al., “Fuselage Leak Detection by Monitoring Cabin Pressure Control System Outflow Valve,” IP.com number000241926, IP.com Electronic Publication Jun. 9, 2015. This method employed monitoring outflow valve positions as the monitored system, looking at ECS and Ventilation System special causes to invalidate the outflow valve monitoring, if required. This description does not account for pressurized air flow through the APU, which adds another degree of freedom to the problem of monitoring the actual fuselage leakage.
As can be seen, there is a need for improved apparatus and methods to monitor and detect excessive fuselage leakage over the in-service life of an airplane and which integrate with the APU, ECS, and CPCS.